The device includes: at leas two ultrasonic transducers, which emit ultrasonic measuring signals into the pipeline and receive ultrasonic measuring signals from the pipeline; and a control/evaluation unit, which ascertains the volume and/or mass flow of the medium in the pipeline on the basis of the travel time difference of the ultrasonic measuring signals in the stream direction and counter to the stream direction. Ultrasonic flow measuring devices are often used in process and automation technology. They make possible contactless determination of volume flow and/or mass flow of a medium in a pipeline.
In the case of the aforementioned travel time difference principle, the different travel times of ultrasonic measuring signals in the stream direction and counter to the stream direction of the medium are ascertained and evaluated. To this end, the ultrasonic measuring signals are alternately emitted by one of the ultrasonic transducers in the flow direction and counter to the flow direction of the medium and, in each case, received by the other of the ultrasonic transducers. From the travel time difference of the ultrasonic measuring signals, the flow velocity can be determined, and with that, at known diameter of the pipeline, volume flow, respectively at known or measured density of the medium, mass flow.
Regarding types of measuring devices, one distinguishes between ultrasonic flow measuring pickups inserted into the pipeline, and clamp-on flow measuring devices, where the ultrasonic sensors are pressed externally onto the pipeline by means of a clamping mechanism. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, and in U.S. Pat. No. 4,484,478 and U.S. Pat. No. 4,598,593.
In the case of both types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated into, and/or received from, the pipeline, or measuring tube, in which the flowing medium is located, at a predetermined angle. In order to achieve an optimum impedance matching, the ultrasonic measuring signals are coupled into the pipeline, respectively out of the pipeline via a transitional member, e.g. a coupling wedge. Main component of an ultrasonic transducer is, furthermore, at least one piezoelectric element, which produces and/or receives the ultrasonic measuring signals.
Usually, the ultrasonic measuring signals used for the volume flow and/or mass flow measurement involve broadband pulses. Of course, exactly in the case of pipelines and measuring tubes of small nominal diameter, the time separation between the emission and reception of the ultrasonic measuring signal is relatively small. In order, in such case, to be able to achieve a sufficient resolution and, therefore, to be able to perform a reliable measurement, the measuring signal is sampled with a sampling rate, which, on the one hand, is smaller than the time duration lying between the emitting and receiving of an ultrasonic measuring signal and which, on the other hand, is sufficiently small that, within the measuring pulse length, a plurality of sample values are sampled. The sampling rate is, as a result, relatively high. The sampled values, respectively the sampled amplitude values, of the ultrasonic measuring signal are fed to an A/D converter. A control/evaluation unit, e.g. a DSP, uses the sampled values to interpolate the received measuring signal by a continuous function, respectively to reconstruct it as close as possible to what it really is. Mathematically, this subject matter can be represented by the continuous function f(t)=f(n·T)=an, wherein n=1, 2, 3, . . . , thus a natural number, and wherein the coefficients αn represent the amplitude values of the ultrasonic measuring signal measured at the points in time (n·T).
In the simplest case, the function is a successive, linear connection of, in each case, two sample values following one after the other. Since this method is not sufficient for measurements of increased accuracy in the field of ultrasonic flow measurement, it is known to apply the Lagrange interpolation, or the yet more complex interpolation of Levenberg-Markart, for the reconstruction of the received measuring signal.
The best and, indeed, most exact theoretical interpolation method for signals reconstructed from sampled values lies, without doubt, in the use of the Shannon-Nyquist Theorem, according to which a bounded, continuous function, e.g. an ultrasonic measuring pulse, can be represented by an infinite sum of weighted sinc functions sin (x)/x. The correct formula is as follows, wherein t represents time and T is the time separation between two sampled values:
      f    ⁡          (      t      )        =            ∑              n        =                  -          ∞                            +        ∞              ⁢                  ⁢                  a        n            ·                        sin          ⁡                      [                          π              ⁡                              (                                                      t                    T                                    -                  n                                )                                      ]                                    π          ⁡                      (                                          t                T                            -              n                        )                              
Problematic with the use of this formula is that the reconstruction of the measuring signal is only 100-percent correct, when the number of sampled values is infinite. In order to be able to apply the Shannon-Nyquist Theorem in practice, it is, naturally, necessary to limit the number of sampled, measured values on the upper end. In doing this, it is to be heeded that the number of sampled values be sufficiently large, in order that a sufficient and adequate reconstruction of the received measuring signal can be achieved. An issue here is a smallest possible calculation time, respectively an adequately large capacity of the microprocessor doing the work of the control/evaluation unit. In the case of the currently known evaluation methods for highly accurate flow measurements by means of ultrasound, the energy requirement for the delivery of (‘almost’) real-time measurements is so large, that, here, only four-wire measuring devices seem suitable. The use of so-called low-energy devices, especially two-conductor ultrasonic flow measuring devices, has, so far, not been possible, due to the high energy requirement. As already indicated, the high energy requirement is primarily a result of the required large calculative capacity of the microprocessor, or DSP, as the case may be. Lastly, the high energy requirement is a result of the complex evaluation methods, which are required for highly dynamic measurements (especially in the field of real-time measurement) with high measurement accuracy.